1. Field of the Invention
The invention relates to continuous casting.
2. Description of the Prior Art
The continuous casting process has been used to produce structural shapes having cross sections of non-uniform thickness. An example of such a structural shape is the beam blank which, as seen in a transverse plane, consists of two flanges connected to one another by a web or crosspiece having a smaller thickness than the flanges.
The mold used to continuously cast a structural shape of non-uniform thickness is provided with a casting cavity having a cross-sectional configuration which corresponds to that of the structural shape. The mold is typically elongated with one end serving as an inlet for molten material and the other end serving as an outlet for the structural shape formed from the material.
In use, the mold is placed in a generally vertical orientation with the inlet end facing up and the outlet end facing down. Cooling water is circulated around the external surface of the mold, and a stream of molten material is introduced into the inlet end of the mold. A pool of the molten material forms in the mold and, at the upper surface of the pool which is referred to as the meniscus, the molten material is in direct and intimate contact with the mold. Slightly below the meniscus, solidification begins to occur adjacent to the internal surface of the mold. A skin of solidified material develops around the molten pool and forms a shield between the pool and the mold so that there is no longer direct contact between the molten material and the mold. Furthermore, due to the shrinkage which accompanies solidification, the skin tends to pull away from the mold so that the contact between the skin and the mold is less intimate than that between the molten material and the mold.
The thickness of the skin increases with increasing distance from the meniscus and, since solidification proceeds in a direction from the internal surface of the mold towards the center of the mold, the cross-sectional area of the pool decreases as the skin thickness increases. The molten pool normally does not solidify completely inside the mold and the product withdrawn from the outlet end of the mold thus consists of a molten core within a shell of solidified material. The cast product is sprayed with water after exiting the mold to complete solidification.
During casting, heat flows from the interior to the exterior of the mold via the internal mold surface which can thus be considered to constitute a heat source. On the other hand, the cooled external surface of the mold, where the heat is carried away from the mold, can be considered to constitute a heat sink.
The mold wall has a constant thickness which is sufficiently large to allow the wall to withstand the pressure of the molten pool inside the mold. In the portions of the mold wall which adjoin the area where the thickness of the casting cavity changes as seen in a transverse plane of the mold, the length of the internal mold surface as measured in the transverse plane is greater than the length of the external mold surface as measured in this plane. Thus, in these portions of the mold wall, the heat source has a substantially greater length than the heat sink which is the reverse of the situation in the remaining portions of the mold wall. Consequently, the rate of heat extraction from the mold in the portions of the mold wall where the thickness of the casting cavity changes is less than the rate in the remaining portions of the mold wall.
At the meniscus level, where the molten material is in intimate contact with the internal surface of the mold wall, the reduced rate of heat extraction causes the temperature of the internal mold surface to increase. This temperature increase can lead to erosion of the mold wall. Below the meniscus level, on the other hand, where a skin has developed and is in less intimate contact with the internal mold surface than is the molten material, the reduced rate of heat extraction from the mold causes the skin thickness to increase more slowly. As a result, cracks are formed in the cast product.
It is an object of the invention to improve the rate of heat extraction in an area where a mold undergoes a dimensional change.
The preceding object, as well as others which will become apparent as the description proceeds, are achieved by the invention.
One aspect of the invention resides in a mold for the continuous casting of molten material. The mold comprises an elongated hollow member defining a casting passage which extends longitudinally of the hollow member. The casting passage has an inlet end for the molten material and an outlet end for a continuously cast product formed from the molten material. The casting passage includes a first portion and a second portion, and the second portion of the casting passage varies in at least one dimension and has at least one location where such dimension is smaller than at any location of the first portion. The hollow member comprises a wall which includes a first section partially bounding the first portion of the casting passage and a second section partially bounding the second or varying portion of the casting passage. The second section of the wall has at least one segment with a wall thickness which is less than that at any location of the first section of the wall.
In a mold according to the invention, a casting passage includes a portion having a variable dimension and this varying portion is partially bounded by a section of a wall. Such section includes at least one segment with a thickness which is reduced relative to the thickness of another section of the wall. The reduction in wall thickness has the effect of decreasing the difference in length between the internal surface of the wall segment and the external surface of the wall segment. As noted earlier, the internal surface of a mold wall may be considered to constitute a heat source whereas the external surface may be considered to constitute a heat sink. Accordingly, in a varying portion of a mold according to the invention, the difference in length between the heat source and the heat sink is smaller than that in conventional molds thereby allowing improved heat transfer to be obtained. Inasmuch as only a minor part of the mold wall needs to have a reduced thickness, this result may be achieved with little or no sacrifice in the strength of the mold wall.
Another aspect of the invention resides in a method of making a mold for the continuous casting of molten material. The method comprises the step of forming an elongated hollow member which defines at least part of a casting passage. The casting passage extends in the longitudinal direction of the hollow member and has an inlet end for the molten material and an outlet end for a continuously cast product formed from the molten material. The casting passage includes a first portion and a second portion, and the second portion of the casting passage varies in at least one selected dimension and has a selected location where the selected dimension is smaller than at any location of the first portion of the casting passage. The hollow member includes a wall having a first section which partially bounds the first portion of the casting passage and a second section which partially bounds the second, varying portion of the casting passage. The method further comprises the step of reducing the wall thickness of a selected segment of the second section of the wall to a value less than that at any location of the first section of the wall.
Since the first portion of the casting passage can have fixed dimensions, the first portion of the casting passage may hereinafter be referred to as the fixed portion of the casting passage for convenience. Similarly, inasmuch as the first wall section partially bounding the fixed portion of the casting passage is thicker than the reduced-thickness segment of the second wall section partially bounding the varying portion of the casting passage, the first wall section will hereinafter be referred to as the thicker wall section. On the other hand, the second wall section will be referred to as the thinner wall section.
The step of forming the elongated hollow member may involve shaping the wall of the hollow member so that the varying portion of the casting passage has at least one part at which the variable dimension this portion is a minimum. The step of reducing the thickness of a selected segment of the thinner wall section is then advantageously performed in such a manner that the selected segment adjoins the part of the casting passage at which the variable dimension is a minimum.
The thinner wall section can have another segment which extends from the selected segment of reduced thickness along the part of the casting passage at which the variable dimension is a minimum. The step of reducing the thickness of a selected segment of the thinner wall section may here comprise shaping the other segment of the thinner wall section such that the other segment widens in a direction away from the selected segment of the thinner wall section.
The wall of the hollow member may include an additional section which partially bounds the varying portion of the casting passage such that the varying portion is sandwiched between the additional section and the thinner section of the wall. The method can then comprise the step of reducing the wall thickness of a segment of the additional wall section to a value less than that at any location of the thicker wall section.
The casting passage can have an additional portion, and the additional portion of the casting passage may be situated so that the varying portion of the casting passage is located between the fixed portion and the additional portion. As indicated previously, the varying portion of the casting passage has a selected location at which a selected dimension is smaller than at any location of the fixed portion of the casting passage, and the selected dimension of this selected location is also smaller than at any location of the additional portion of the casting passage. The wall of the hollow member includes another section which partially bounds the additional portion of the casting passage, and the step of reducing the thickness of a selected segment of the thinner wall section may here include decreasing the wall thickness of the selected segment to a value less than that at any location of the other wall section.
Since, like the first portion of the casting passage, the additional portion of the casting passage can have fixed dimensions, the additional portion of the casting passage will hereinafter be referred to as an additional fixed portion for convenience.
The step of forming the hollow member can involve shaping the wall of the hollow member so that at least one part of the varying portion of the casting passage narrows in a direction away from the fixed portion of the casting passage. The shaping may be performed in such a manner that this part of the casting passage narrows substantially continuously in a direction away from the fixed portion of the casting passage.
The step of forming the hollow member can further comprise shaping the wall of the hollow member so that an additional part of the varying portion of the casting passage narrows in a direction towards the fixed portion of the casting passage.
The step of forming the hollow member may also comprise shaping the wall of the hollow member so that the part of the casting passage at which the variable dimension is a minimum is substantially centered with respect to the fixed portion and the additional fixed portion of the casting passage. The wall of the hollow member can be shaped in such a manner that the casting passage resembles a beam having a pair of flanges joined by a web.
The thinner wall section may include at least one additional segment which is located between the thicker wall section and the selected segment of the thinner wall section. The method can here additionally comprise the step of shaping the additional segment of the thinner wall section so that the additional segment narrows in a direction away from the thicker wall section. The shaping of the additional segment may be performed in such a manner that the additional segment narrows substantially continuously in a direction away from the thicker wall section.
The additional segment of the thinner wall section can be disposed to one side of the part of the casting passage at which the variable dimension is a minimum and the thinner wall section may have a second additional segment on an opposite side of this part of the casting passage. The step of reducing the thickness of a selected segment of the thinner wall section can then comprise shaping the second additional segment so that the latter narrows in a direction towards the thicker wall section.
The step of forming the hollow member may involve shaping the thinner wall section so that this wall section defines at least one concavity as seen from externally of the hollow member.
The thicker wall section has at least one location with a predetermined wall thickness which is less than or equal to the wall thickness at all other locations of the thicker wall section. Also, the selected segment of the thinner wall section includes a first surface portion which faces the casting passage and a second surface portion which faces away from the casting passage. The steps of forming the hollow member and reducing the thickness of the selected segment of the thinner wall section can here comprise: (a) shaping the first surface portion to produce an arc which has a first radius and is concave as seen from externally of the hollow member; and (b) shaping the second surface portion to produce an arc which has a second radius smaller than the first radius and is concave as seen from externally of the hollow member. The steps of forming the hollow member and reducing the thickness of the selected segment of the thinner wall section are carried out in such a way that the second radius differs from the first radius by less than the predetermined wall thickness of the thicker wall section.
The step of forming the hollow member may be performed such that the thicker wall section has a substantially constant wall thickness.